Compressor Repair checks for problems that could be preventing Compressor from operating correctly. It checks for missing files, incorrect permissions, an invalid hostname, and attempts to manually start the qmasterd process.

Apple's Compressor relies heavily on the Qmaster service in order to work. Qmaster allows you to split rendering tasks across multiple machines in a render farm but it is still required even if you are only rendering on a single computer.

The link between the two is so important that if the Qmaster service fails to launch, Compressor will be unable to submit batches.